In the name of the Father and of the Son and of the Holy Spirit

Matthew 28:16-20
When they saw him, they worshiped him; but some doubted.
Jesus came to them and said, "All authority in heaven and on earth has been given to me. Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age."
The eleven remaining apostles return to Galilee. There Jesus, now possessing 'all authority', gives them his last command - to make disciples of all nations.
All through his Gospel Matthew has referred back to the Jewish scriptures, showing how Jesus fulfilled them. But there is nothing exclusive about the good news. And the final words of the Gospel are universal: God's kingdom is open, the invitation is extended, to people of every nation.
Why?
Father, Son and Holy Spirit : this phrase became a standard expression in the formal liturgy(禮儀) of the early church.
Here is the word of Trinity comes from.
Some disciple doubted Jesus has risen, who is Thomas.
Last command is a blessing to all who obey his word. Jesus is with us always.
How?
This is the last command to all his followers, preach Gospel to all nations and baptizing them and teaching them to obey Jesus have commanded us.